Add a hook to allow extensions to prevent HTML file caching

Needed for MobileFrontend on third-party sites that use this feature.
Ideally, there should be a way to cache mobile requests too, but that's
a different issue requiring much more effort to do properly.

Bug: 68106
Change-Id: I01a76c571d9186b325f19a00cec136459707c791
Merge "Add a hook to allow extensions to prevent HTML file caching"